There’s no more Mr. and Mrs. Nice Guy in the Massachusetts U.S. Senate race.

After Democratic Harvard Law Professor Elizabeth Warren released a TV ad attacking Republican U.S. Sen. Scott Brown, Brown is fighting back with a negative ad of his own – focusing on Warren’s use of her Native American heritage.

The ad features a series of news clips from this summer when Warren was confronted with questions about whether she used her Native American heritage to advance her career. The ad concludes with a clip of Warren being asked whether anything else will come out about her. Her joking response, “I don’t think so, but who knows?”

Warren listed her herself as a minority in law directories, and Harvard University and the University of Pennsylvania both listed her as Native American in diversity statistics. Warren has not provided documentation of her heritage, but has said it is part of her family lore.

The Boston Globe recently reported that Warren’s family members have mixed memories about their Native American heritage. While none provided documentation, several family members told stories that echoed Warren’s recollections, while others were unfamiliar with the claims.

The topic proved somewhat damaging to Warren in May and June. A Suffolk University poll done in May, in the midst of the controversy, found that 69 percent of voters did not believe Warren’s Native American heritage was a significant story – and voters generally believed Warren was telling the truth about her heritage. But among those who were aware of the controversy, Warren had a higher unfavorable rating than among those who were not aware of it. Brown also led by a larger margin among those voters.

Until recently, Brown had shied away from overtly attacking Warren on the issue. However, when the topic came up in last week’s feisty Senate debate, Brown used it to attack Warren’s character.

“When you are a U.S. Senator, you have to pass a test, and that’s one of character and honesty and truthfulness. I believe…she’s failed that test,” Brown said.

Warren said she heard stories about her Native American heritage growing up. Warren said she did not use her heritage to get into college or law school, and those who hired her did not know about her heritage until she was hired.

A pledge that Brown and Warren signed preventing outside groups from buying TV ads has meant that the campaigns must take direct responsibility for their messages. Until recently, most of the advertising in the race had been positive. Warren released the first negative TV ad on Sept. 13, accusing Brown of siding with big corporations and “big money.”

Then, Brown criticized Warren for negative advertising. “It's unfortunate that instead of running a campaign that lifts people up, Elizabeth Warren has decided to run a campaign that tears people down," Brown said in a statement.

Warren released another ad last week criticizing Brown’s voting record.